1. Field of the Invention
The present invention relates to a fuel-cells system including fuel cells that receive supplies of gaseous fuel and oxidizing gas and generate an electromotive force, and also to a method of regulating the temperature in the fuel-cells system.
2. Description of the Prior Art
Fuel cells (hereinafter may be referred to as FC) receive a supply of gaseous fuel containing at least hydrogen on an anode thereof and a supply of oxidizing gas containing at least oxygen on a cathode thereof and generate an electromotive force through electrochemical reactions. Equations (1) through (3) given below represent electrochemical reactions proceeding in the fuel cells. Equation (1) shows the reaction proceeding at the anode, whereas Equation (2) shows the reaction proceeding at the cathode. The reaction shown by Equation (3) accordingly proceeds as a whole in the fuel cells. EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- (1) EQU (1/2)O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O (2) EQU H.sub.2 +(1/2)O.sub.2 .fwdarw.H.sub.2 O (3)
The fuel cells receive a supply of fuel and convert the chemical energy of the fuel directly into electrical energy at a high energy efficiency. In the actual operation of the fuel cells, however, the above electrochemical reactions do not proceed at the efficiency of 100%. Part of the chemical energy of the fuel is not taken out as the electrical energy but is released as heat to the outside. A fuel-cells system including such fuel cells accordingly requires a structure that removes the heat produced during the operation of the fuel cells and keeps the driving temperature of the fuel cells in a predetermined temperature range.
Cooling water is generally used to remove the heat produced during the operation of the fuel cells. The fuel cells are constructed as a stack structure in which a plurality of unit cells are laid one upon another. A flow path of a predetermined configuration is formed in the stack structure. Cooling water is flown through the flow path to remove the heat produced in the process of the electrochemical reactions and keep the driving temperature of the fuel cells in a predetermined temperature range. In a known fuel-cells system (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 6-188013), a heat exchange unit, such as a radiator, cools down the hot cooling water that has been flown through the flow path in the fuel cells to remove the heat produced in the process of the electrochemical reactions. The cold cooling water after the heat exchange is again fed to the flow path in the fuel cells. This structure prevents the temperature in the fuel cells from increasing excessively and enables the driving temperature of the fuel cells to be kept to or below a predetermined level.
The quantity of heat produced in the fuel cells depends upon the degree of the electrochemical reactions proceeding in the fuel cells and the efficiency of power generation in the fuel cells. In the prior art fuel-cells system, the on-off state of a cooling fan included in the radiator for cooling down the cooling water is controlled to change the cooling state of the cooling water (that is, the cooling power) with a variation in quantity of heat. The cooling water is circulated between the fuel cells and the radiator by means of a pump. When the internal temperature of the fuel cells (or the temperature of the cooling water corresponding to the internal temperature of the fuel cells) is not higher than a predetermined level, the cooling fan is turned off to stop the positive cooling process of the cooling water. When the internal temperature of the fuel cells becomes greater than the predetermined level, on the other hand, the cooling fan is turned on to start the positive cooling process of the cooling water. This procedure prevents the driving temperature of the fuel cells from exceeding the predetermined level.
The temperature control carried out in the prior art fuel-cells system can not sufficiently equalize the temperatures of the respective unit cells, however, while enabling the driving temperature of the fuel cells to be kept to or below the predetermined level. Namely, the prior art temperature control can not ensure the performance of the fuel cells sufficiently. Even when the mean driving temperature of the stack of fuel cells is not higher than the predetermined level, the unit cells constituting the stack of fuel cells include those having relatively high temperatures and those having relatively low temperatures. This causes the respective unit cells to have different output voltages. The following describes the relationship between the driving temperature of the fuel cells and the output voltage.
FIG. 11 is a graph showing the output voltage of a fuel cell plotted against the driving temperature of the fuel cell when the output current density of the fuel cell is kept constant. The fuel cell has an optimum driving temperature Ta that gives the maximum output voltage as shown in the graph of FIG. 11. In the case that a supply of gaseous fuel fed to the fuel cell has a sufficient concentration of steam, the optimum driving temperature Ta is substantially kept constant even when the magnitude of loading connected to the fuel cell varies to some extent. For example, the optimum driving temperature of polymer electrolyte fuel cells is approximately 80.degree. C. Each unit cell included in the stack of fuel cells has this relationship between the driving temperature and the output voltage. In order to improve the performance of the fuel cells, it is desirable that the driving temperature of each unit cell included in the stack of fuel cells is always kept to a constant value, which is as close as possible to the optimum driving temperature Ta.
The distribution of the temperatures of the respective unit cells included in the stack of fuel cells is ascribed to the difference in the progress of electrochemical reactions proceeding in the respective unit cells, as well as to the effects of the outside temperature and to the temperature gradient between an inlet and an outlet of cooling water in the fuel cells. The prior art structure that changes over the on-off state of the cooling fan to control the positive cooling process of the cooling water, which is circulated between the fuel cells and the radiator, can not sufficiently equalize the temperatures of the respective unit cells at a desired level. Any proposed structures can not equalize the temperatures of the respective unit cells included in the stack of fuel cells in a desirable temperature range.